Abstract

This module describes the fragmentation model that partitions a flow into pages, columns, or regions.
It builds on the Page model module and introduces and defines the fragmentation model.
It adds functionality for pagination, breaking variable fragment size and orientation, widows and orphans.

CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.

Status of this document

This section describes the status of this document at the time of its publication.
Other documents may supersede this document.
A list of current W3C publications and the latest revision of this technical report
can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document was produced by the CSS Working Group (part of the Style Activity)
as a Candidate Recommendation. This document is intended to become a W3C Recommendation.
This document will remain a Candidate Recommendation at least until 1 May 2017 in order
to ensure the opportunity for wide review.

GitHub Issues are preferred for discussion of this specification.
When filing an issue, please put the text “css-break” in the title,
preferably like this:
“[css-break] …summary of comment…”.
All issues and comments are archived,
and there is also a historical archive.

Publication as a Candidate Recommendation does not imply endorsement by the W3C
Membership. This is a draft document and may be updated, replaced or
obsoleted by other documents at any time. It is inappropriate to cite this
document as other than work in progress.

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

Breaking the Web, one fragment at a time

1. Introduction

This section is not normative.

In paged media (e.g., paper, transparencies, photo album pages, pages
displayed on computer screens as printed output simulations), as
opposed to continuous media, the content of the document is split into one or
more discrete display surfaces. In order to avoid awkward breaks
(such as halfway through a line of text), the layout engine must be
able to shift around content that would fall across the page break.
This process is called pagination.

1.1. Module Interactions

1.2. Values

This specification follows the CSS property definition conventions from [CSS21]. Value types not defined
in this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS
modules may expand the definitions of these value types: for example [CSS3VAL],
when combined with this module, adds the initial value
to the properties defined here.

In addition to the property-specific values listed in their definitions, all properties
defined in this specification also accept the inherit keyword as their property value. For readability it has not been
repeated explicitly.

2. Fragmentation Model and Terminology

fragmentation container (fragmentainer)

A box—such as a page box, column box, or region—that contains
a portion (or all) of a fragmented flow.
Fragmentainers can be pre-defined, or generated as needed.
When breakable content would overflow a fragmentainer in the block dimension,
it breaks into the next container in its fragmentation context instead.

The portion of a box that belongs to exactly one fragmentainer.
A box in continuous flow always consists of only one fragment.
A box in a fragmented flow consists of one or more fragments.
Each fragment has its own share of the box’s border, padding, and margin,
and therefore has its own padding area, border area,
and margin area.
(See box-decoration-break, which controls how these are affected by fragmentation.)

Each fragmentation break (hereafter, break)
ends layout of the fragmented box in the current fragmentainer and causes the remaining content to be laid out in the next fragmentainer,
in some cases causing a new fragmentainer to be generated
to hold the deferred content.

Breaking inline content into lines is another form of fragmentation,
and similarly creates box fragments when it breaks inline boxes across line boxes.
However, inline breaking is not covered here; see [CSS21]/[CSS3TEXT].

2.1. Parallel Fragmentation Flows

When multiple formatting contexts are laid out parallel to each other,
fragmentation is performed independently in each formatting context.
For example, if an element is floated,
then a forced break inside the float
will not affect the content outside the float
(except insofar as it may increase the height of the float).
UAs may (but are not required to)
adjust the placement of unforced breaks in parallel formatting contexts to visually balance such side-by-side content,
but must not do so to match a forced break.

The following are examples of parallel flows
whose contents will fragment independently:

The contents of a float vs. the content wrapping outside the float.

The contents of a float vs. the contents of an adjacent float.

The contents of each table cell in a single table row.

The contents of each grid item in a single grid row.

The contents of each flex item in a flex layout row.

The contents of absolutely-positioned elements
that cover the same range of their containing block’s fragmentation context.

Content overflowing the content edge of a fixed-size box
is considered parallel to the content after the fixed-size box
and follows the normal fragmentation rules.
Although overflowing content doesn’t affect the size of the fragmentation root box,
it does increase the length of the fragmented flow,
spilling into or generating additional fragmentainers as necessary.

For example, if a region box is broken at a page boundary,
then the content of the region will be affected by a page break at that point
(but not by a region break).

Note that when a multi-column element breaks across pages,
it generates a new row of columns on the next page for the rest of its content,
so that a page break within a multi-column element
is always both a page break and a column break.

3. Controlling Breaks

The following sections explain how breaks are controlled in a fragmented flow.
A page/column/region break opportunity between two boxes
is under the influence of
the containing block’s break-inside property,
the break-after property of the preceding element,
and the break-before property of the following element.
A page/column/region break opportunity between line boxes
is under the influence of
the containing block’s break-inside, widows, and orphans properties.
A fragmentation break can be
allowed, forced, or discouraged depending on the values of these properties.
A forced break overrides any break restrictions acting at that break point.
In the case of forced page breaks, the author can also specify
on which page (left or right)
the subsequent content should resume.

See the section on rules for breaking for the exact rules on how these properties affect fragmentation.

Since breaks are only allowed between siblings,
not between a box and its container (see Possible Break Points),
a break-before value on a first-child box is propagated to its container.
Likewise a break-after value on a last-child box is propagated to its container.
(Conflicting values combine as defined below.)
This propagation stops before it breaks through the nearest matching fragmentation context.

Values for break-before and break-after are defined in the sub-sections below.
User Agents must apply these properties to boxes in the normal flow of the fragmentation root.
User agents should also apply these properties to floated boxes
whose containing block is in the normal flow of the root fragmented element.
User agents may also apply these properties to other boxes.

Generic Break Values

These values have an effect regardless of
the type of fragmented context containing the flow.

Force one or two page breaks before/after the principal box so that the next page is formatted as a left page.

right

Force one or two page breaks before/after the principal box so that the next page is formatted as a right page.

recto

Force one or two page breaks before/after the principal box so that
the next page is formatted as either a left page or a right page,
whichever is second (according to the page progression) in a page spread.

verso

Force one or two page breaks before/after the principal box so that
the next page is formatted as either a left page or a right page,
whichever is first (according to the page progression) in a page spread.

Column Break Values

These values only have an effect in multi-column contexts;
if the flow is not within a multi-column context, they have no effect.

The orphans property specifies the minimum number
of line boxes in a block container
that must be left in a fragmentbefore a fragmentation break.
The widows property specifies the minimum number
of line boxes of a block container
that must be left in a fragmentafter a break.
Examples of how they are used to control fragmentation breaks are given below.

Only positive integers are allowed as values of orphans and widows.
Negative values and zero are invalid and must cause the declaration to be ignored.

If a block contains fewer lines than the value of widows or orphans,
the rule simply becomes that all lines in the block must be kept together.

To guarantee progress, fragmentainers are assumed to have a minimum block size of 1px regardless of their used size.

4.1. Possible Break Points

Fragmentation splits boxes in the block flow dimension.
In block-and-inline flow, breaks may occur at the following places:

Class A

Between sibling boxes of the following types:

Block-parallel Fragmentation

When the block flow direction of the siblings' containing block
is parallel to that of the fragmentation context: in-flow block-level boxes,
a float and an immediately-adjacent in-flow or floated box,
table row group boxes,
table row boxes,
multi-column column row boxes.

Block-perpendicular Fragmentation

When the block flow direction of the siblings' containing block
is perpendicular to that of the fragmentation context:
table column group boxes,
table column boxes,
multi-column column boxes.

Class B

Between line boxes inside a block container box.

Class C

Between the content edge of a block container box and the outer edges of its
child content (margin edges of block-level children or line box
edges for inline-level children) if there is a (non-zero)
gap between them.

There is no inherent prioritization among these classes of break points.
However, individual break points may be prioritized or de-prioritized
by using the breaking controls.

Other layout models may add breakpoints to the above classes.
For example, [CSS3-FLEXBOX] adds certain points within a flex formatting context
to classes A and C.

Some content is not fragmentable, for example many types of replaced elements[CSS21] (such as images or video),
scrollable elements, or a single line of text content.
Such content is considered monolithic:
it contains no possible break points.
Any forced breaks within such boxes
therefore cannot split the box,
and must therefore also be ignored by the box’s own fragmentation context.

4.2. Types of Breaks

A break between two page boxes that are not associated with facing pages.
A spread break is always also a page break. [CSS3PAGE]

column break

A break between two column boxes.
Note that if the column boxes are on different pages, then the break is
also a page break.
Similarly, if the column boxes are in different regions,
then the break is also a region break. [CSS3COL]

When multiple forced break values apply to a single break point,
they combine such that all types of break are honored.
When left, right, recto, and/or verso are combined,
the value specified on the latest element in the flow wins.

A forced page break must also occur at a class A break point if the last line box above this margin and the first one below it do not have the
same value for page. See [CSS3PAGE]

When a forced break occurs,
it forces ensuing content into the next fragmentainer of the type associated with the break,
breaking through as many fragmentation contexts as necessary
until the specified break types are all satisfied.
If the forced break is not contained within a matching type of fragmentation context,
then the forced break has no effect.

4.4. Unforced Breaks

While breaking controls can force breaks,
they can also discourage them.
An unforced break is one that is inserted automatically by the UA
in order to prevent content from overflowing the fragmentainer.
The following rules control whether unforced breaking
at a possible break point is allowed:

However, if all of them are auto and a common ancestor of all
the elements has a break-inside value of avoid, then breaking
here is not allowed.

Rule 3

Breaking at a class B break point is allowed only if the number
of line boxes between the break and the start of the enclosing block
box is the value of orphans or more, and the number of line boxes
between the break and the end of the box is the value of widows or more.

If the above doesn’t provide enough break points to keep content from
overflowing the fragmentainer,
then rule 3 is dropped to provide more break points.

If that still does not lead to sufficient break points,
then rules 1, 2 and 4 are dropped in order to find additional breakpoints.
In this case the UA may use the avoids that are in effect at those
points to weigh the appropriateness of the new breakpoints; however,
this specification does not suggest a precise algorithm.

Finally, if there are no possible break points below the top of the fragmentainer,
and not all the content fits, the UA may break anywhere
in order to avoid losing content off the edge of the fragmentainer. In such cases, the UA may also fragment the contents of monolithic elements
by slicing the element’s graphical representation.
However, the UA must not break at the top of the page,
i.e. it must place at least some content on each fragmentainer,
so that each fragmentainer has a non-zero amount of content,
in order to guarantee progress through the content.

4.5. Optimizing Unforced Breaks

While CSS3 requires that a fragmented flow must break at allowed
break points in order to avoid overflowing the fragmentainers in its
fragmentation context, it does not define whether content breaks
at a particular allowed break.
However, it is recommended that user agents observe the following
guidelines (while recognizing that they are sometimes contradictory):

Break as few times as possible.

Make all fragmentainers that don’t end with a forced break appear
to be equally filled with content.

Avoid breaking inside a replaced element.

Suppose, for example, that the style sheet contains orphans : 4, widows : 2, and there is space for 20 lines (line boxes) available
at the bottom of the current page, and the next block in normal flow
is considered for placement:

If the block contains 20 line boxes or fewer, it should be placed
on the current page.

If the block contains 21 or 22 line boxes, the second fragment of
the paragraph must not violate the widows constraint, and so
the second fragment must contain at least two line boxes;
likewise the first fragment must contain at least four line boxes.

If the block contains 23 line boxes or more, the first fragment should
contain 20 lines and the second fragment the remaining lines. But if
any fragment of the block is placed on the current page, that fragment
must contain at least four line boxes and the second fragment at least
two line boxes.

Now suppose that orphans is 10, widows is 20, and there
are 8 lines available at the bottom of the current page:

If the block contains 8 lines or fewer, it should be placed
on the current page.

If the block contains 9 lines or more, it must NOT be split
(that would violate the orphans constraint), so it must
move as a block to the next page.

5. Box Model for Breaking

The sizing terminology used in this section is defined in [CSS3-SIZING].

5.1. Breaking into Varying-size Fragmentainers

When a flow is fragmented into varying-size fragmentainers, the following
rules are observed for adapting layout:

Intrinsic sizes are calculated and maintained across the entire element.
Where an initial containing block size is needed to resolve an intrinsic
size, assume the size of the first fragmentainer defining a fragmentation context.

Layout is performed per-fragmentainer, with each fragmentainer continuing
progress from the breakpoint on the previous, but recalculating sizes
and positions using its own size as if the entire element were fragmented
across fragmentainers of this size. Progress is measured in percentages
(not absolute lengths) of used/remaining fragmentainer extent and in amount of
used/remaining content.
However, when laying out monolithic elements,
the UA may instead maintain a consistent inline size and resolved block size across fragmentainers.

Fragments of boxes that began on a previous fragmentainer must obey
placement rules with the additional constraint that fragments must
not be positioned above the block-start edge of the fragmentainer.
If this results in a box’s continuation fragment
shifting away from the block-start edge of the fragmentainer, then box-decoration-break: clone, if specified, wraps the fragment
with the box’s margin in addition to its padding and border.

Illustration of breaking in varying-size fragmentainers.

Since document order of elements doesn’t change during fragmentation,
fragments are processed following the same rules that apply to continuous
media. In particular, the order of floats is preserved across all
fragments and follows the same rules as defined in CSS 2.1 9.5.

A block-level continuation fragment may be placed below the top of
the page if, e.g. it establishes a block formatting context and
is placed beside a float and both it and the float continue onto
a narrower page that is too narrow to hold both of them side-by-side.

An element adjacent to a preceding float on one page may wind up above
the float’s continuation on the next page if,
e.g. that float is pushed down because
it no longer fits side-by-side with an earlier float
that also continues onto this narrower page.

A left float may appear on a page before the remaining fragments of a preceding right float
if that right float does not fit on the earlier page.
However another right float will be forced down until
the preceding right float’s remaining fragment can be placed.

Here is an example that shows the use of percentage-based progress:
Suppose we have an absolutely-positioned element that is positioned top: calc(150% + 30px) and has height: calc(100% - 10px). If
it is placed into a paginated context with a first page height of 400px,
a second page of 200px, and a third page of 600px, its layout progresses
as follows:

First, the top position is resolved against the height of the first page.
This results in 630px. Since the first page has a height of only 400px, layout
moves to the second page, recording progress of 400/630 = 63.49% with
36.51% left to go.

Now on the second page, the top position is again resolved, this
time against the height of the second page. This results in 330px. The
remaining 36.51% of progress thus resolves to 120.5px, placing the top edge of
the element 120.5px down the second page.

Now the height is resolved against the second page; it resolves
to 190px. Since there are only 79.5px left on the page, layout moves
to the third page, recording progress of 79.5/190 = 41.84%, with 58.16%
left to go.

On the third page, the height resolves to 590px. The remaining
58.16% of progress thus resolves to 343.1px, which fits on this page
and completes the element.

5.2. Adjoining Margins at Breaks

When an unforced break occurs between block-level boxes,
any margins adjoining the break truncate to the remaining fragmentainer extent before the break,
and are truncated to zero after the break.
When a forced break occurs there, adjoining margins before the break are truncated,
but margins after the break are preserved. Cloned margins are always truncated to zero on block-level margins.

The extra block size contributed by fragmenting the box
(i.e. the distance from the break point to the edge of the fragmentainer)
contributes progress towards any specified limits on the box’s block size.

Each box fragment is independently wrapped with the border, padding, and margin.
The border-radius and border-image and box-shadow, if any,
are applied to each fragment independently.
The background is drawn independently in each fragment of the element.
A no-repeat background image will thus be rendered once in each fragment of the element.

The effect is as though the element were rendered with no breaks present,
and then sliced by the breaks afterward:
no border and no padding are inserted at a break;
no box-shadow is drawn at a broken edge;
and backgrounds, border-radius, and the border-image are applied to the geometry of the whole box
as if it were unbroken.

UAs may also apply box-decoration-break to control rendering
at bidi-imposed breaks, i.e. when bidi reordering causes an inline
to split into non-contiguous fragments. Otherwise such breaks are
always handled as slice.

For inline elements, which side of a fragment is considered the broken edge
is determined by the parent element’s inline progression direction.
For example, if an inline element whose parent has direction: rtl breaks across two lines, the left edge of the fragment on
the first line will be the broken edge. (Note in particular that
neither the element’s own direction nor its containing block’s direction is used.) See [CSS3-WRITING-MODES].

For box-decoration-break: slice, backgrounds (and border-image)
are drawn as if applied to a composite box
consisting of all of the box’s fragments
reassembled in visual order.
This theoretical assembly occurs after the element has been laid out
(including any justification, bidi reordering, page breaks, etc.).
To assemble the composite box...

For boxes broken across lines

First, fragments on the same line are connected in visual order.
Then, fragments on subsequent lines are ordered
according to the element’s inline base direction and aligned on the element’s dominant baseline.
For example, in a left-to-right containing block (direction is ltr),
the first fragment is the leftmost fragment on the first line
and fragments from subsequent lines are put to the right of it.
In a right-to-left containing block,
the first fragment is the rightmost on the first line
and subsequent fragments are put to the left of it.

For boxes broken across columns

Fragments are connected as if the column boxes were glued together
in the block flow direction of the multi-column element.

For boxes broken across pages

Fragments are connected as if page content areas were glued together
in the block flow direction of the root element.

If the box fragments have different widths
(heights, if the fragments are joined horizontally),
then each piece draws its portion of the background
assuming that the whole element has the same width (height) as this piece.
However, if the used height (width) of an image is derived from the width (height) of the box,
then it is calculated using the widest fragment’s width
and maintained as a fixed size.
This ensures that right-aligned images stay aligned to the right edge,
left-aligned images stay aligned to the left edge,
centered images stay centered,
and stretched images cover the background area as intended
while preserving continuity across fragments.

5.5. Transforms, Positioning, and Pagination

Fragmentation interacts with layout, and thus occurs before relative positioning [CSS21], transforms [CSS3-TRANSFORMS],
and any other graphical effects.
Such effects are applied per fragment:
for example, rotation applied to a fragmented box
will calculate a rotation origin for each fragment
and independently rotate that fragment around its origin.
(The origin of an overflow-only fragment is determined
as if that content were overflowing an empty box with zero margins/borders/padding
at the start of the fragmentainer.)
However, in order to reduce dataloss when printing,
the separation and transfer of page boxes should occur last;
thus a transformed fragment that spans pages should be sliced at the page breaks and print in its entirety
rather than being clipped by its originating page.

A fixed-height box spanning 2.5 pages with overflow content spanning
to a total of 4 pages.
The transform origin of each fragment is the center of its border box;
the fragment without a border box assumes a zero-height box at the start of the overflow.

Absolute positioning affects layout and thus interacts with fragmentation.
Both the coordinate system and absolutely-positioned boxes belonging to a containing block
will fragment across fragmentainers in the same fragmentation flow as the containing block.

UAs are not required to correctly position boxes that span a fragmentation break and whose block-start edge position depends on where the box’s content fragments.

UAs with memory constraints that prevent them from manipulating an entire document in memory
are not required to correctly position absolutely-positioned elements
that end up on a previously-rendered page.

Acknowledgments

The editors would like to thank
Mihai Balan,
Michael Day,
Alex Mogilevsky,
Shinyu Murakami,
Florian Rivoal,
and Alan Stearns
for their contributions to this module.
Special thanks go to the former [CSS3PAGE] editors
Jim Bigelow (HP),
Melinda Grant (HP),
Håkon Wium Lie (Opera),
and
Jacob Refstrup (HP)
for their contributions to this specification,
which is a successor of their work there.

Conformance

Document conventions

Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.

All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with class="example",
like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the
normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <strong class="advisement">, like
this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification
is defined for three conformance classes:

A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.

A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements
for implementing CSS responsibly,
in a way that promotes interoperability in the present and future.

Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid
(and ignore as appropriate)
any at-rules, properties, property values, keywords, and other syntactic constructs
for which they have no usable level of support.
In particular, user agents must not selectively ignore
unsupported property values and honor supported values in a single multi-value property declaration:
if any value is considered invalid (as unsupported values must be),
CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage,
implementers should release an unprefixed implementation
of any CR-level feature they can demonstrate
to be correctly implemented according to spec,
and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.

CR exit criteria

For this specification to be advanced to Proposed Recommendation,
there must be at least two independent, interoperable implementations
of each feature. Each feature may be implemented by a different set of
products, there is no requirement that all features be implemented by
a single product. For the purposes of this criterion, we define the
following terms:

independent

each implementation must be developed by a
different party and cannot share, reuse, or derive from code
used by another qualifying implementation. Sections of code that
have no bearing on the implementation of this specification are
exempt from this requirement.

interoperable

passing the respective test case(s) in the
official CSS test suite, or, if the implementation is not a Web
browser, an equivalent test. Every relevant test in the test
suite should have an equivalent test created if such a user
agent (UA) is to be used to claim interoperability. In addition
if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those
equivalent tests in the same way for the purpose of
interoperability. The equivalent tests must be made publicly
available for the purposes of peer review.

implementation

a user agent which:

implements the specification.

is available to the general public. The implementation may
be a shipping product or other publicly available version
(i.e., beta version, preview release, or "nightly build").
Non-shipping product releases must have implemented the
feature(s) for a period of at least one month in order to
demonstrate stability.

is not experimental (i.e., a version specifically designed
to pass the test suite and is not intended for normal usage
going forward).

The specification will remain Candidate Recommendation for at least
six months.